It is known that polyisocyanates can be reacted with higher molecular weight polyols such as polyesters, polyethers, polycarbonates and/or polyamines as well as chain lengthening agents such as low molecular weight polyols and/or polyamines, optionally in the presence of water, blowing agents, catalysts, emulsifiers, stabilizers and other auxiliary agents such as fillers and pigments, to produce various high molecular weight polyurethane or polyurea resins depending on the formulations employed. The products obtained are suitable for use as elastomers, foam resins, foils or adhesives depending on their engineered properties.
In all of the products obtained, the physical properties are primarily determined by so-called "hard segments" (blocks of urethane or urea groups) and secondarily by the "soft segments" (residues of higher molecular weight polyols or polyamines). The mechanical properties of the polyurethane resins produced from the usual starting components, for example their tensile strength, elongation at break, elasticity and compression resistance, are unsatisfactory for many purposes. A substantial improvement is generally obtained by including among the starting materials higher molecular weight polyhydroxyl compounds in which various synthetic resins (polymers, polycondensates or polyisocyanate polyaddition products) are dispersed in finely divided form. Processes of this kind have been described, for example, in German Offenlegungsschriften Nos. 1,769,869; 2,014,385; 2,324,134 and 2,423,984 and in U.S. Pat. Nos. 2,993,013 and 3,869,413.
Even in these improved synthetic resins, however, the tendency of the polyurethanes to show undesirable deviations in their elastic properties under long term stress, in particular a marked permanent elongation, is a disadvantage which still exists. For this reason, such polyurethane resins are quite unsuitable for use as tires, for example, because the centrifugal forces would progressively loosen the structure of the polymer so that the tire would eventually burst.
It would be an obvious expedient to overcome this undesirable tendency of polyurethanes to creep by increasing the degree of chemical cross-linking, of example by using an excess of isocyanate or by using trifunctional starting components. However, an increase in the degree of cross-linking is accompanied by a sharp increase in the brittleness of polyurethanes. This manifests itself in a marked drop in tensile strength, dimensional stability and elongation at break. Furthermore, the preparation of prepolymers from polyfunctional starting compounds is accompanied by a sharp rise in viscosity and frequently also by coagulation.